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Sucking carbon dioxide from air too costly, say physicists

Mopping carbon dioxide directly from the sky costs too much be an economically viable way to mitigate global warming, a two-year study by 13 physicists has found (press release). One method, using reactive chemicals, would cost at least $600 to remove each tonne of CO2 from the air – around eight times more pricey than capturing the gas from the industrial flue stacks of power plants, the report [pdf] concludes.

The study – published by the American Physical Society on 9 May – doesn’t put a figure on the costs of innovative technologies from researchers and private companies who claim to be able to capture CO2 from air more cheaply.

“They proved that one specific way to capture carbon dioxide from air is expensive,” says Klaus Lackner, at Columbia University in New York, who has developed one of the new technologies. But that, he says, does not mean other methods are not worth pursuing. “If you study penguins, you might jump to the conclusion that birds can’t fly.”

Princeton University physicist Robert Socolow, who co-chaired the study, says the report could not evaluate the costs of other technologies, because there was no verifiable data to draw on. “We didn’t want to try to come up with premature numbers and then have an argument,” he says.

The basic concept of taking CO2 from the air is alluring and simple. Air-capture devices could be put anywhere on the planet, and could extract an unlimited amount of the gas, directly cutting down on atmospheric CO2 concentrations. A crude chemical system that involves passing air over sodium hydroxide (also known as lye), and then using further chemical reactions and a hot kiln to separate the CO2 and sorbent for re-use, has been known for 70 years. (Nicola Jones’s Nature feature, ‘Sucking it up’, has the details).

The catch is that this system is very energy-intensive, and incredibly costly. Essentially, the chemical sorbent grabs onto the CO2 so tightly that it takes a lot of energy to prise the stuff off again. What the APS has done, says Socolow, is provide the first transparent and fully documented analysis of the cost of a real-world system. It ends up quite similar to earlier ball-park estimates (‘Sucking it up’ quotes an estimate of $500/tonne CO2).

In 2007, billionaire Richard Branson and former US vice-president Al Gore offered up to $25 million for the first demonstrably viable commercial design to remove significant amounts of CO2from the atmosphere. No-one has yet claimed the prize, but a number of researchers say they are working on technologies which could slash costs of commercial air carbon-capture. Lackner’s resin-based system uses swings of humidity to capture and release CO2. His work is being commercialised by Kilimanjaro Energy, based in New York and San Francisco, and his visions of landscapes of ‘artificial trees’ sucking up CO2 get frequent media coverage. “I think we can get into the game at around $200 per tonne of CO2, and drop the price from there,” he says.

Another contender is the company Global Thermostat, co-founded in 2006 by physicist Peter Eisenberger – who founded Columbia University’s Earth Institute – and economist Graciela Chichilnisky. The team uses amine solvents to grab CO2 and recycles the sorbent using low-temperature steam (waste heat from other industrial processes). It has a pilot plant at SRI International, in Menlo Park, California.

The APS report makes reference to these systems – indeed, 30 of its 119 pages are devoted to discussing how to improve the air carbon-capture process. But it doesn’t try to quantify their prospects. “Our report doesn’t say ‘slam the door’,” points out Socolow. “In the review process, most people said we needed to be more negative.”

Ultimately, says Michael Desmond, a chemist at BP who co-chaired the report, the study shows that carbon capture from air is very low down the pecking order of technologies that should be supported by public research funds. Tomorrow, the Senate Energy and Natural Resources Committee holds a hearing on a bill that would provide incentives for ‘capturing carbon dioxide from dilute sources … using direct air capture technologies’.

On his blog, Roger Pielke, a climate policy expert at the University of Colorado in Boulder, says “There is little point in debating specific details about costs or large-scale feasibility until demonstration projects are undertaken”. And he adds: “As far as dueling cost estimates for technologies that do not exist? Call me a skeptic, but I’d prefer to evaluate technologies based on how they perform in the real world, not in expert reports or even peer reviewed papers.”


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    barry said:

    The time-tested way to strip CO2 from the atmosphere is through photosynthesis. Right now, about 20 times as much CO2 is stripped from the atmosphere each day as all human contribution. What we need to do is merely to keep some of that biomass from cycling back into the atmosphere through cellulases in the soil and in the guts of termites and ungulates. “Antropogenic peat” or “biochar” are cheap feasible technologies today.

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    jean nutson said:

    sucking carbon dioxide from the atmosphere biologically is the most cost effective method,this can be done simply by planting more trees or increasing the vegetative cover of the planet for the chlorophyll to absorb the Carbon dioxide for their food production which in turn serves as the primary food source in the food chain.

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